Dispersion Strengthened Copper (“DSC”) is typically supplied in extruded and drawn shapes including rounds, rectangles, wire, plates, tubes, and rolled strip. DSC is a thermally stable copper, which retains a high percentage of its strength at elevated temperatures. DSC does not recrystallize or soften after exposure to temperatures approaching the melting point of copper. Since DSC uses inert aluminum oxide particles to strengthen the copper matrix, the thermal conductivity of the copper is not significantly decreased.
DSC is used for applications requiring electrical and thermal conductivity near that of pure copper, while maintaining strength and resistance to softening at elevated temperatures. The commercial applications for DSC include resistance welding electrodes, incandescent lamp lead wires, diodes, X-Ray and microwave tube components, high temperature contacts and particle accelerator components. DSC resistance welding electrodes offer improved life due to reduced weld tip surface softening and non-sticking properties when welding coated steels.
The “canister method” is an example of DSC manufacturing, which begins with a canister made of pure copper. The canister is cleaned with an acid or alkaline cleaner to remove machining lubricants or any oxides from the surface. DSC powder is then placed into the canister to achieve a fill factor of between 50 to 55 percent. The canister can then be purged with nitrogen if desired and evacuated and sealed. This process of evacuating the canister creates a vacuum inside the canister to eliminate trapped gas and minimize expansion during preheating of the canister powder prior to extrusion. Heat can also be applied to the canisters during filling while creating the vacuum to remove any trapped moisture in the assembly. Once the vacuum has been established and moisture removed the canister is then sealed. After sealing the canister the assembly is referred to as a billet. The billet containing DSC powder is formed into rod by first preheating the billet to a given temperature and then extruding. Since the wall thickness of the canister is small (for example, less than 5 millimeters), a typical DSC rod may have a cross section of 92% DSC.
The drawbacks to the canister method include: fabrication of a pure copper canister that adds no additional functional benefit to the final product, limited fill factor of powder into the canister creating low yields, the time required to evacuate a canister, increased heating time to heat an assembly prior to extrusion due to the separation between powder particles that reduces conduction heating, and an amount of DSC needed for the end product due to the small wall thickness.
The fill factor (actual weight of powder/theoretical full density weight for the given volume) of DSC powder and evacuation of the canister affects quality of the DSC rod. Consequently, other methods, such as Hot Isostatic Pressing (“HIP”) or Cold Isostatic Pressing (“CIP”) have been used to consolidate DSC powder into billets that are then extruded into a rod. These methods rely upon fluid pressure to increase the density of DSC. However, HIP and CIP have extreme costs associated with their operation. In particular, estimated cost per HIPing load using a large Quintus chamber would be $7,000 to $8,000 and having the capacity for 7 k to 11 k pounds of DSC billets. This would equate to approximately $0.833/lb. and would require a 24-hour cycle. The cost for CIPing is approximately $500 per item and takes approximately one hour to complete. The estimated cost for CIPing a DSC billet would be $2.00/lb. In contrast, the cost of the canister method can be between $250-$500 per item. Additionally, a disadvantage to HIP is that the assembled canisters containing DSC powder need to be heated to remove moisture, crimp sealed and welded, leak tested, and then HIPed. For powder the main obstacles related to HIPing is that the fill factor must be greater than 50% prior to HIP. If the fill factor is less than 50% then the powder must be CIPed prior to being placed into a canister.
One use of DSC is in resistance welding electrodes. Resistance welding has long been used as a quick and effective method of joining metal members. The workpieces to be welded are placed in abutting relationship and a large current is caused to flow through the workpieces by a pair of opposed electrodes. The current causes the abutting surfaces of the workpieces to be heated sufficiently to effect the formation of a weld nugget. Typically, the electrodes apply significant pressure to the workpieces during welding. This facilitates the welding process by urging the material together and, also, reducing electrical resistance between each electrode tip and the adjacent workpiece material.
Since welding is accomplished by resistance heating of the material being welded, it will be appreciated that the electrodes will also be heated substantially. It is important to have an electrode of high electrical conductivity in order to minimize the power loss in the electrode and the resulting heating of the electrode.
Over time, the repeated heating and pressing operations involved in resistance welding cause breakdown, softening, mushrooming and other deformation of the electrodes. As this occurs, electrical current requirements increase with the enlarged welding tip face contacting the workpiece material until ultimately, redressing or replacement of the electrode is required. Accordingly, it is also important to have an electrode which is capable of withstanding significant distorting force at the elevated temperatures which result from the welding process so as to minimize the number of times it becomes necessary to redress or replace the electrode within a given period of time.
It is known in the art to form resistance welding electrodes by combining a copper electrode body with an anneal resistant high hardness insert. Typically, the insert performs much better than the copper material from which the electrode body was formed. However, the insert material is much more expensive than the copper used to form the electrode body.
The insert may be brazed onto the shank. The brazing step is disadvantageous as it adds an additional step to the electrode manufacturing process and, hence, increases the cost of the electrode. Furthermore, the brazing operation may anneal and soften the electrode body.
It is also known to force the insert into an electrode body via a press-fit operation. The steel welded today is often galvanized, or coated with a zinc or other softer metal coating. As a result, the resistance welding electrodes tend to stick to the coated metal. An electrode tip joined to an electrode body via a press-fit operation may pull out of the shank during resistance welding of coated materials, thus requiring replacement of the electrode.